Improvements of circuit performance in the IGFET technology are readily achieved by reducing both horizontal and vertical geometry. Advantages due to reductions of horizontal dimensions are sometimes negated by the mask tolerances needed in order to insure corrective alignment between consecutive masking steps. Consequently, it is desirable to use processes with self-alignment features and a minimum number of masking steps. Reducing the number of masking steps also improves yields and cuts down manufacturing costs.
The prior art has disclosed a three mask self-aligned process for making an IGFET in C. C. Mai, et al., Three Mask Self-Aligned MOS Technology, IEEE Transactions on Electron Devices, ED-20, December 1973, pages 1162-1163. The reference disclosed a three mask MOS process using an aluminum gate. This process produces a device having the disadvantage that the aluminum gate does not cover the thin oxide layer overlapping the source and drain diffusions. Experience has shown that a mere misalignment of a metal gate with respect to the oxide taper in the gate region can reduce the product reliability by a factor of ten. However in the case of the device produced by the process disclosed in the C. C. Mai, et al. reference, it is not really a question of misalignment but of no thin oxide coverage at all. The reliability of the device produced by the referenced process would be objectionable in any long term application. Ionic contamination of the gate region would be the failure mode.
Existing processes for producing polycrystalline silicon gate, IGFET devices are typified by M. Faggin, et al., "Silicon Gate Technology", Solid State Electronics, Vol. 13, 1970, Pgs. 1125-1144. Faggin, et al., shows a four mask process with a first mask defining the field region, the second mask defining a polycrystalline silicon gate, source and drain regions, the third mask to open windows for source, drain contacts, and a fourth mask defining the metallized contact to the device. Since each mask introduces an additional horizontal tolerance in the position of the elements of the device, the corresponding production yield of a four mask process is highly sensitive to the precision with which the successive masks are mutually aligned. A reduction in the number of masks would improve the production yield of the resulting product.